1. Field of the Invention
The present invention relates to a road surface gradient detecting apparatus for use in a vehicle such as a motor vehicle in which wheels are driven by an engine through a power transmission device having interposed therein a torque converter and relates, in particular, to a road surface gradient detecting apparatus which is suitable in performing a control of a starting frictional engaging element which is interposed in series with the torque converter inside the power transmission device, the control being performed depending on the road surface gradient at the time when the vehicle is at a standstill.
2. Description of Related Art
Conventionally, as a control apparatus for a starting frictional engaging element, the following is known. Namely, even in a particular driving state in which an engine is idling and the vehicle speed is below a predetermined value, the starting frictional engaging element is arranged to be capable of being engaged so that the vehicle can move in the form of a so-called creep running. In addition, in order to avoid the occurrence of vibrations of a vehicle body and poor fuel economy due to a creeping torque at the time of depressing a brake pedal, the engaging force of the starting frictional engaging element at the time of a vehicle standstill with the brake pedal being depressed is made smaller than the one when the brake pedal is not being depressed (see, for example, Published Unexamined Japanese Patent Application No. 216842/1987 and Published Unexamined Japanese Patent Application No. 244930/1989).
In the above-described arrangement, when the brake pedal is released, the engaging force of the starting frictional engaging element is increased, and the creeping torque returns to an original magnitude. The engaging force of the starting frictional engaging element, however, does not increase instantaneously, with the result that a slight time lag occurs for the creeping torque to return to the original magnitude.
When the vehicle starts on an upward slope (or on an upgrade), if the time lag occurs, as described above, to the returning of the creeping torque after releasing the depression of the brake pedal, it becomes necessary for the driver to quickly depress the accelerator pedal right after releasing the depression of the brake pedal to obtain a torque which counters a backward-moving torque of the vehicle due to gravity.
In order to improve the starting operability (or the ease with which the vehicle can start) on an upgrade, there is known an arrangement in which a means for detecting a road surface gradient is provided. When the upgrade has a road surface gradient exceeding a predetermined value, the decrease in the engaging force of the starting frictional engaging element is prohibited when the brake is in operation (see Published Unexamined Japanese Patent Application No. 210093/1997).
In addition, there is also known an arrangement in which a road surface gradient is calculated from a wheel driving force and a vehicle acceleration, although it is a road surface gradient detecting apparatus for speed change control in an automatic transmission (see Published Unexamined Japanese Patent Application No. 207735/1997).
As a road surface gradient detecting means for controlling a starting frictional engaging element, it is considered to use the above-described road surface gradient detecting apparatus, instead of a special sensor. This detecting apparatus, however, is arranged to calculate the wheel driving force based on an engine output torque and has therefore the following disadvantages. Namely, the actual engine output torque varies with aging of the engine, the environment in which the engine is used, the kind of fuel to be used, or the like. Therefore, the wheel driving force cannot be calculated accurately, resulting in an error in detecting the road surface gradient.
Further, in the region of low rotational speed of the engine, the variation in the engine output torque becomes large. Therefore, if the wheel driving force is calculated based on the engine output torque in the region of low rotational speed of the engine such as at the time of deceleration (or speed reduction) which leads to a standstill (or stopping) of the vehicle, the error becomes larger. As a result, there is a possibility that the control of the starting frictional engaging element depending on the road surface gradient at the time of vehicle standstill cannot be performed appropriately.
In case a torque converter is interposed in the power transmission device, the fluid transmission torque TQ of the torque converter is expressed as
TQ=Kxc3x97xcexaxc3x97"xgr"xc3x97N2
where xcexa is a torque ratio of the torque converter, xcfx84 is an input shaft torque coefficient, N is an input rotational speed of the torque converter, and K is a coefficient peculiar to the torque converter. Based on this toque TQ, the wheel driving force can be calculated. In case of a torque converter containing therein a lock-up clutch, if the input side and the output side of the toque converter are directly coupled together by the operation of the lock-up clutch, the output torque of the converter becomes equal to the engine output torque which corresponds to the input torque of the torque converter. However, at the time of speed reduction leading to the vehicle standstill, the lock-up clutch becomes inoperative and, thus, the output torque of the torque converter can be calculated by the above-described formula.
In view of this point, the present invention has an object of providing a road surface gradient detecting apparatus which is capable of accurately computing the road surface gradient.
In order to attain the above and other objects, the present invention is a road surface gradient detecting apparatus for use in a vehicle in which wheels are driven by an engine through a power transmission device having interposed therein a torque converter containing therein a lock-up clutch, the road surface gradient being calculated by a wheel driving force and a vehicle acceleration, the apparatus comprising: a first driving force calculating means for calculating, while the lock-up clutch is not in operation, the wheel driving force based on that fluid transmission torque of the torque converter which is obtained by a speed ratio of the torque converter and an input rotational speed of the torque converter; a second driving force calculating means for calculating, while the lock-up clutch is in operation in a direct-coupled state, the wheel driving force based on an engine output torque; and a third driving force calculating means for calculating, while the lock-up clutch is in operation in a slipping state, the wheel driving force based on a total torque of that fluid transmission torque of the torque converter which is obtained by the speed ratio of the torque converter and the input rotational speed of the torque converter and that transmission torque of the lock-up clutch which is calculated by an engaging force of the lock-up clutch, whereby the road surface gradient is calculated by the wheel driving force which is calculated by the driving force calculating means corresponding to the state of the lock-up clutch.
According to the present invention, when the lock-up clutch has becomes inoperative, the wheel driving force is calculated by the first driving force calculating means based on that fluid transmission torque of the torque converter which is calculated by the torque ratio of the torque converter, the input shaft torque coefficient, and the input rotational speed of the torque converter, according to the above-described formula, the toque ratio of the torque converter being searched with the speed ratio of the torque converter serving as a parameter. By using this wheel driving force, the road surface gradient is calculated. Here, the fluid transmission torque to be calculated by the above-described formula is hardly subject to the effect of deterioration with aging, or the like. Therefore, the wheel driving force can be accurately calculated whereby an accuracy of calculating the road surface gradient can be secured.
When the lock-up clutch is in operation in a direct-coupled state, the wheel driving force is calculated by the second driving force calculating means based on the engine output torque. On the other hand, when the lock-up clutch is in operation in a slipping state, the wheel driving force is calculated by the third driving force calculating means based on the total torque of the fluid transmission torque of the torque converter and the transmission torque of the lock-up clutch. At the time of speed reduction of the vehicle leading to the vehicle standstill, since the operation of the lock-up clutch is switched from the direct-coupled state to the slipping state or the inoperative state, the wheel driving force is calculated by the third driving force calculating means or the first driving force calculating means. Therefore, even if the engine output torque largely varies in the region of low rotational speed of the engine, the wheel driving force can be calculated without using the engine output torque. Based on this wheel driving force, the road surface gradient can be accurately calculated. In this manner, the starting frictional engaging element can be accurately controlled depending on the road surface gradient at the time of the vehicle standstill.
In case of a torque converter which does not contain therein a lock-up clutch, only the first driving force calculating means is provided. The road surface gradient is then calculated by using the wheel driving force to be calculated based on that fluid transmission torque of the torque converter which is obtained by the speed ratio of the torque converter and the input rotational speed of the torque converter.